In a wireless mesh network, a node can have several neighbours with which to communicate. In such a network, pairs of neighbouring nodes arrange times during which to communicate and the duration of such meetings. This meeting duration is sometimes referred to as a slot time. When the meeting duration for communicating with a given neighbour has expired, a node then switches to communicate with its next neighbour. The time to cycle through all of the neighbours is called a cycle time.
Communications between nodes typically includes information for negotiating the start time and duration of a next meeting as well as uplink and downlink data transmission. It is beneficial to minimize the time required to perform the negotiation.
If the data transfer is unidirectional at a given time, the initiating node of the negotiation sends its negotiation information in a negotiation request message. The node responding to the request then sends its negotiation information back to the initiating node. This is then followed by a confirmation message by the initiating node. This process is known as a 3-way handshaking process.
If the data transfer is bidirectional, there are only two messages. The initiating node sends its negotiation information. The responding node then determines the next meeting time and duration based on the negotiation information sent by the initiating node and sends confirmation of the determined meeting time and duration to the initiating node. In this case, the negotiation process is a 2-way handshaking process.
In a particular negotiation process that is the result of applying 802.11 contention mechanisms to mesh networks the entire negotiation process is performed before the start of communication between the nodes within a given slot time. FIG. 1 will now be used to illustrate a known negotiation method of communication between two neighbouring nodes for one slot time 100. For each pair of neighbouring nodes there is a downlink (DL) and an uplink (UL) communication. While nodes of a mesh network are not typically considered to have DL and UL directionality based on the connectivity of a mesh, in this context the DL direction is used to refer to the direction of the initial negotiation request and the UL direction is used to refer to the direction of the negotiation confirmation or response.
Contention mechanisms in IEEE 802.11 are used by nodes in a network to obtain a communication medium over which the nodes will communicate. When there are multiple nodes, the nodes need to contend for the medium to determine which node can use the communication medium at a particular time and for a particular duration. Since the whole point of the negotiation process is to schedule a subsequent time for two nodes to communicate, it may be expected that only these two nodes would be contending for the communication medium at the subsequent scheduled time. However, it is possible that there are other nodes in proximity of the two nodes with the scheduled meeting time that are not privy to the fact that the two nodes have established a scheduled meeting time. Not being privy to such information of a scheduled meeting, these other nodes may attempt to obtain the communication medium for their own use.
A typical slot time includes a DL contention and negotiation portion 110, a UL contention and negotiation portion 120, a DL contention and data transmission portion 130 and a UL contention and data transmission portion 140. During downlink contention and negotiation 110, different nodes contend for the downlink channel. Only once a given node wins out in the contention process does that node then go ahead with the negotiation. Each DL contention involves waiting for a Distributed Coordination Function (DCF) Interframe Space (DIFS) 111 and RBO (random backoff) 112 to ensure that no other communication is occurring at that time on the channel being requested. Because multiple nodes may be contending for the medium during this time, and these nodes may be transmitting packets that collide with each other, the time taken to access the channel can take up a significant portion of the available slot. The illustration only shows a single contention request. However, if there is detected communication, the node continues the contention process until it has successfully obtained the medium. Once the channel is successfully accessed, this is followed by the transmission of a negotiation request (NEG) 113 and the later receipt of an acknowledgement (ACK) 114.
Similarly, during uplink contention and negotiation portion 120, different nodes contend for the uplink channel. Only once a given node wins out in the contention process does that node then go ahead with the negotiation. Each UL contention attempt includes the waiting duration of DIFS 121 and RBO 122. Because multiple nodes may be contending for the medium during this time, and these nodes may be transmitting packets that collide with each other, the time taken to access the channel can take up a significant portion of the available slot. The illustration only shows a single contention request. Once the channel is successfully accessed, this is followed by the transmission of a negotiation response (NEG) 123 and the later receipt of an acknowledgement (ACK) 124.
Once the DL contention and negotiation and UL contention and negotiation are complete, any remaining time left in the scheduled slot is available for data transmission during the DL contention and data transmission portion 130 and the UL contention and data transmission portion 140. However, each data packet transmitted during this remaining part of the slot is also subject to contention for accessing the medium. This is notwithstanding the fact that during the previous slot, a time and duration for both UL and DL transmission were negotiated. The reason for this is that 802.11 employs a contention based mechanism that requires contention for each and every access. It was not specifically designed for mesh networks.
Since each node must contend for the medium before any negotiation packet is transmitted, the total time needed to perform the negotiation is not bounded. The time needed to contend for the medium depends on DIFS and on a contention window size. The RBO is based on the contention window. In this case, it is possible that the negotiation process can exceed the maximum slot time. This leads to an unbounded cycle time and therefore to a delay in delivering delay sensitive packets such as voice.
Existing negotiation methods require many attempts at contending for the medium during a meeting. The methods are not efficient in terms of data transmission and have a low probability of successfully completing the negotiation in the allocated time.